Dear Tolunay,
in message 445B8086.9000404@orkun.us you wrote:
This patch would solve the issue that exists today that when the "active" environment is lost/corrupted for some reason the "redundant" environment would contain an exact copy of the primary to have the board come up without requiring the need to redo the changes that was lost on
Actually I think that you will not acchieve this with your patch. This is why I'm concerned. You see, if you feel better having this patch I would not complain, but I am afraid that a lot of people might just activate it because they think it would do them any good when it doesn't (and actually it just hurts).
There is only one occasion when we have any significant likelyhood of losing the environment data: this is when a call to "saveenv" fails becaue either a) we have a power loss, b) we have an otherwise induced reset of the CPU, or c) the flash sector that shall be erased/written is failing.
So where exactly does your modification improve anything? Let's go through this step by step.
Case 1: power loss/reset happens during the first "saveenv", i. e. when writing the first copy of the new environment data.
In this case this first copy contains no valid data; the second copy of the environment contains valid, but old data.
This is exactly the same as we have with the current imple- mentation. I don't see any improvement.
Case 2: power loss/reset happens during the second "saveenv", i. e. when writing the second copy of the new environment data.
In this case this first copy contains valid new data, while the second copy of the environment does not contain valid data.
In the current implementation, the first (and only) saveenv would have completed, too, and the reset would hit after leaving this part of code, so we had valid new data in the first copy, and valid (but old) data in the second one.
Again, this is not an improvement. Actually I think the current implementations is even more useful.
Case 3: A flash sector in the first copy of the environment becomes defective while we erase or write it. In this case we will see appropriate error conditions, and the "saveenv" command will abort.
This is the same as case 1: no valid data in copy 1, valid, but old data in copy 2; no difference between the existing and your new implementation.
Case 4: A flash sector in the second copy of the environment becomes defective while we erase or write it. In this case we will see appropriate error conditions, and the "saveenv" command will abort.
This is the same as case 2: valid new data in copy 1, no valid data in copy 2 with your implementation, but probably valid old data with the existing code.
I guess I must have missed some cases because there was none yet where the new implementaion would improve the reliability. Please fill in these missing cases.
But, and I think this is an undisputet fact, the current implemen- tation needs only hald the number of erase/write cycles, so it causes much less flash wear than your code. [Actually your code will see the same level of flash wear as you have now without the redundant environment enabled; it's that enabling the current implementation of redundance *improves* flash lifetime by halfing the number of erase/write cycles to the environment.]
Among the things that can cause one environment to go corrupt would be charge decays in memory cells in aging flash, supply variations/noise
I think that the likelyhood of such a thing to happen during read accesses only is infinitesimal.
during erase/write and random memory corruption when power is
I agree that erase/write cycles are the critical phase where corruption may happen, and which we want to try to protect with our implementation. See above.
interrupted while another section of flash memory is being written/erased.
I don't see how this could happen to flash. [Well, I've seen flash corruption before; this was on Intel flash where you could write the flash control commands to arbitrary addresses, so just copying a binary image to a flash device could cause random write / erase actions. But then, such devices should have hardware flash protection (which you should enable, or you deserve what you get), or if you are concerned about reliability you would avoid such devices like hell.]
Sure these could cause other problems as well like if this issue happens for U-Boot code the system might become un-bootable. But at least we have full recovery for the case when it happens within U-Boot environment.
I'm not sure I can follow that logic. If you have some undetected and unexpected memory corruption in your flash, and if you care about reliability, then you must try to recognize such situations and halt the system. Trying to continue in such an undefined state is too hazardous.
So, can you please fill in the szenario where your modification would really help to make the system more reliable?
Best regards,
Wolfgang Denk